DOF Cards for Minolta 7000i?

Can anyone clue me in on how these exactly work. Is it a readout thing or does it actually stop the lens down?

In the same line, if you've shot the camera what did you think about it. I was given one and am unsure whether I should invest any time and money in it. The 7000i viewfinder is so big compared to my Maxxum 70 that I can practically see to focus without my glasses. Is there another body with the same size screen but with DOF preview available in a non-chintzy plastic for a reasonable price?